Allow the kernel to build with a compiler that sets -fno-common.

[brooks]

Summary

The mechanism that generates assym.inc and offset.inc depends on the
symbols in question being common. For now, simply force the object files
to be created with -fcommon.

[kevans]

I took a brief look at rewriting the awk to work with either -fcommon or -fno-common a lttle bit ago, but that's non-trivial as things don't line up as neatly.

[brooks]

The values are also quite different if you diff the nm output. I think this whole thing will need to be rethought eventually, but removing -fcommon is probably a long ways off.